This invention relates generally to high speed electro-optic analog-to-digital converters, and, more specifically, to such analog-to-digital converters in which the quantization of the analog signal is performed entirely by passive photonic elements. This invention also relates to the field of integrated photonic structures in which such electro-optical analog-to-digital converters may be monolithically fabricated.
Analog-to-digital converters ("ADCs") have traditionally been fabricated using microelectronics. For low-speed (i.e., low-bandwidth) analog signals, microelectronic based ADCs are adequate. A problem arises where the analog signal to be digitized is broadband or where it must be sampled at the carrier's frequency. Under these circumstances, microelectronic ADCs quickly approach the limits of their performance. Furthermore, while the speed and resolution of microelectronic ADCs have evolved over the years, this evolution is slow--e.g., six years of development may yield only one bit of improvement in the resolution of a microelectronic ADC. (Robert H. Walden, "Analog-to-Digital Converter Survey and Analysis", 17 IEEE Journal on Selected Areas in Communications, 539-550 (April 1999)). Unfortunately, military radar and electronic warfare systems require more than what the state of the art can provide in microelectronic ADCs.
It is thus desirable to provide faster analog-to-digital conversion at higher resolution. Optical sampling provides a significant improvement in ADC performance. In conventional microelectronic ADCs, conversion speed (interchangeable with bandwidth) must be traded for resolution (the length in bits of the representative digital word). With microelectronics, it is simply not possible to achieve both wide bandwidth and high resolution. Optical ADCs, however, overcome this constraint by rapidly converting broadband analog signals to highly resolved digital representation (i.e., words with many bits).
In an optimal analog-to-digital conversion, i.e., with wide bandwidth and high resolution, most of the conversion takes place on a modulated light signal with a maximum of optical processing. Though the prior art tried this approach, it has failed to make use of the full extent of optical processing. Specifically, the prior art still relegates to microelectronics the quantization step of analog-to-digital conversion. Thus the prior-art optical ADCs are suboptimal.
Both time-division multiplexing (TDM) and wavelength-division multiplexing (WDM) optical ADCs exist in the prior art. They are physically large, complicated branched structures that are very sensitive to optical path length and temperature effects. They require precise timing to reconstitute signals. Most importantly, they require microelectronic ADCs to quantize and digitize the sampled analog signals that emanate from each optical branch. Though they do sample a wide band analog signal by dividing it into a set of discrete optical signals, both TDM and WDM optical ADCs require microelectronic ADCs to complete the analog-to-digital conversion.
U.S. Pat. No. 5,010,346 to Hamilton et al. discloses a mode-locked laser source divided into a plurality of optical signals, each of which is modulated by an analog signal. This plurality of modulated optical signals is detected optically and amplified. Hamilton et al. requires that the modulated optical signal be split as many times as is necessary to ensure that the resultant bandwidth of the "split" signal falls within the speed and resolution capability of the microelectronic quantization portion of the circuit. Accordingly, Hamilton, et al. teaches a microelectronic ADC to quantize and digitize each and every one of the plurality of detected outputs. Thus the apparatus of Hamilton et al. ceases to be optical downstream of the detectors. Therefore the resolution and conversion speed limitations typical of microelectronic ADCs limits the utility of Hamilton et al.'s apparatus for broadband signal applications.
U.S. Statutory Invention Registration USH0000353 to Taylor teaches pairs of optical waveguides on a substrate. A modulator induces either of two possible orthogonal phases that represent the amplitude of the sampled analog signal. Optical phase detectors produce one of two possible least significant bit ("LSB") states (i.e., "1" or "0") that depends on the phase state detected. Taylor teaches how to process LSB representations to form a complete digital word that represents the sampled analog signal. Taylor neither teaches nor suggests a means by which a quantized digital word can be formed without digital processing, and therefore, without slowing the analog-to-digital conversion by the time required for digital processing. Therefore the performance of Taylor's apparatus on broadband signals is inherently extremely limited.
U.S. Pat. No. 4,325,603 to Marom also teaches parallel optical waveguides disposed on a substrate wherein laser light is coupled between optical waveguide pairs. The amount of coupling is proportional to the analog signal applied. Marom teaches further that the length of the coupling region between waveguide pairs can be predetermined. Thus Marom eliminates the need for phase shifting the laser light (and the need for light polarizers to obtain bi-phase light). Marom requires, however, two optical waveguide channels for each single bit state determination (i.e., "1" or "0") and two comparators to determine the relative intensity of the waveguides.